Chlorine dioxide has found wide use as a disinfectant in water treatment/purification, as a bleaching agent in pulp and paper production, and a number of other uses due to its high oxidizing power. There is a variety of chlorine dioxide generator systems and processes available in the marketplace. Most of the very large scale generators employed, for example, in pulp and paper production, utilize an alkali metal chlorate salt, a reducing agent, and an acid in a chemical process for producing chlorine dioxide. These generators and the processes employed also produce by-product salts such as sodium chloride, sodium sulfate, sodium sesquisulfate or sodium bisulfate. In pulp and paper mills, the typical by-product is sodium sulfate (saltcake) which is converted into a sulfur salt of sodium in a high temperature boiler and used in the paper process. Boilers require energy and the paper mills have a limited boiler capacity. Increasing the production of chlorine dioxide generally means increased capital investment to provide the added boiler capacity required to process the added amounts of saltcake by-product produced.
Thus a process which reduces the amount of a by-product salt, such as sodium chloride or sodium sulfate, produced while efficiently generating chlorine dioxide is commercially desirable.
U.S. Pat. No. 3,810,969 issued May 14, 1974 to A. A. Schlumberger teaches a process for producing chloric acid by passing an aqueous solution containing from 0.2 gram mole to 11 gram moles per liter of an alkali metal chlorate such as sodium chlorate through a selected cationic exchange resin at a temperature from 5.degree. to 40.degree. C. The process produces an aqueous solution containing from 0.2 gram mole to about 4.0 gram moles of HClO.sub.3. This process requires the regeneration of the cationic exchange resin with acid to remove the alkali metal ions and the treatment or disposal of the acidic salt solution.
K. L. Hardee et al, in U.S. Pat. No. 4,798,715 issued Jan. 17, 1989, describe a process for chlorine dioxide which electrolyzes a chloric acid solution produced by passing an aqueous solution of an alkali metal chlorate through an ion exchange resin. The electrolyzed solution contains a mixture of chlorine dioxide and chloric acid which is fed to an extractor in which the chlorine dioxide is stripped off. The ion exchange resin is regenerated with hydrochloric acid and an acidic solution of an alkali metal chloride is formed.
In U.S. Pat. No. 4,683,039, Twardowski et al describe a method for producing chlorine dioxide in which the chlorine dioxide is produced in a generator by the reaction of sodium chlorate and hydrochloric acid. After separating chlorine dioxide gas the remaining sodium chloride solution is fed to a three-compartment cell to form sodium hydroxide and an acidified liquor which is returned to the chlorine dioxide generator.
Each of the above processes produces a fixed amount and type of by-product salt.
M. Lipsztajn et al, teach an electrolytic-dialytic process for producing chloric acid and sodium hydroxide from sodium chlorate. Chlorate ions are transferred through an anion-exchange membrane and sodium ions are passed through a cation-exchange membrane (U.S. Pat. No. 4,915,927, Apr.10, 1990).
M. Lipsztajn et al, in PCT Application No. WO 92/03374, published Mar. 5, 1992, teach an electrochemical process for producing chlorine dioxide comprising the reduction of chloric acid in an aqueous reaction medium in a reaction zone at a total acid normality of up to about 7 normal and in the substantial absence of sulfate ion and in the presence of a dead load of alkali metal chlorate added to and subsequently removed from the reaction medium. The dead load of alkali metal chlorate cycles between a reaction zone and an electrolysis zone. In the process, the electroysis zone comprises an electrolytic cell comprising a cation-exchange membrane dividing the cell into an anode compartment and a cathode compartment, feeding a chloric acid solution with a dead load of alkali metal chlorate into the anode compartment, electrolytically producing hydrogen ions in the anode compartment while simultaneously effecting transfer of alkali metal cations from said anode compartment through said cation-exchange membrane to said cathode compartment, and removing an acidified feed solution from said anode compartment.
Applicants have found when operating similar processes where acidic solutions containing chlorate ions are electrolyzed in an anode compartment, that in addition to the oxidation of water to produce oxygen and hydrogen ions, chlorate ions are also oxidized at the anode forming perchlorate ions. These perchlorate ions as perchloric acid in the acidic solutions in a chlorine dioxide generator also can react with added reducing agents, but does not form chlorine dioxide. This formation of perchlorate ion creates an inefficiency, resulting in the added consumption of valuable chlorate and reducing agent and thereby increases the cost of producing chlorine dioxide.